1. Field of the Invention
The invention relates to an image processing method, an image processing device, and an image processing system and, more particularly, to an image processing method, an image processing device, and an image processing system having a dynamic range compression processing function which are suitable for use in an image process, an image collection, or the like of an x-ray chest image.
2. Related Background Art
For example, since an x-ray chest image is constructed by an image of a lung sac where an x-ray can easily transmit and an image of a mediastinum where it is very difficult for the x-ray to transmit, a range where pixel values exist is very wide.
Therefore, it has been considered that it is difficult to obtain an x-ray chest image in which both of the lung sac and the mediastinum can be simultaneously observed.
To diagnose the chest by a doctor, therefore, there is a case where an x-ray image (film) for diagnosis of the lung sac and an x-ray image (film) for diagnosis of the mediastinum are individually photographed and prepared.
As a method of avoiding such a problem, therefore, there is a method called a "self compensation digital filter".
The self compensation digital filter is expressed by the following equations (1) and (2). EQU S.sub.D =S.sub.org +f(S.sub.US) (1) EQU S.sub.US =.SIGMA.S.sub.org /M.sup.2 (2)
where, S.sub.D :pixel value after the compensation (after the process)
S.sub.org :original pixel value (input pixel value) PA1 S.sub.US :average pixel value when a moving average is obtained from the original image (input image) with respect to a mask size of (M.times.M pixels) PA1 f(x):function having characteristics as shown in FIGS. 1A and 1B PA1 S.sub.org : pixel value of the input image PA1 S.sub.US : average pixel value when a moving average is obtained from the input image with respect to a mask size of (M.times.M pixels) PA1 f(): function to control a processing effect BASE: threshold value to limit a processing range; and PA1 subsequently, executing a contrast enhancing process which is expressed by the following arithmetic operating equation EQU S.sub.c =S.sub.d +expand.sub.-- coeff.times.g(S.sub.d -BASE); S.sub.d &lt;BASE=S.sub.d ; S.sub.d .gtoreq.BASE PA1 S.sub.d :dynamic range compression pixel value expand.sub.-- coeff:coefficient to control a ratio of contrast enhancement PA1 BASE:threshold value PA1 g():function to control a processing effect. PA1 S.sub.org : pixel value of the input image PA1 S.sub.US : average pixel value when a moving average is obtained from the input image with respect to a mask size of (M.times.M pixels) PA1 f(): function to control a processing effect PA1 BASE: threshold value to limit a processing range; and PA1 contrast enhancing means for executing a contrast enhancing process which is expressed by the following arithmetic operating equation EQU S.sub.c =S.sub.d +expand.sub.-- coeff.times.g(S.sub.d -BASE); S.sub.d &lt;BASE=S.sub.d ; S.sub.d .gtoreq.BASE PA1 S.sub.d : pixel value of a compression image obtained by said dynamic range compressing means PA1 expand.sub.-- coeff: coefficient to control a ratio of contrast enhancement PA1 BASE:threshold value PA1 g():function to control a processing effect to the compression image obtained by the dynamic range compressing means
The characteristics which the function f(x) has will now be described. First, the characteristics shown in FIG. 1A are as follows. That is, now assuming that x is a signal value and BASE.sub.a is a threshold value, f(x) is equal to "0" when "x&gt;BASE.sub.a ". When "0.ltoreq.x.ltoreq.BASE.sub.a ", f(x) monotonously decreases while setting a slice to the "threshold value BASE.sub.a " and an inclination to "SLOPE.sub.a ". (Hereinafter, the function f(x) having the above characteristics is shown by "f.sub.a (x)".)
Therefore, when the above equation (1) is executed on the assumption that the original pixel value S.sub.org is set to a density correspondence amount, an effect for an image such that a density is enhanced at a position where the average density of the image is low, is derived.
On the other hand, the characteristics shown in FIG. 1B are as follows. That is, now assuming that the signal value is labelled to x and a threshold value is set to BASE.sub.b, f(x) is equal to "0" when "0.ltoreq.x&lt;BASE.sub.b ". When "x .gtoreq.BASE.sub.b ", f(x) monotonously decreases to a negative region while setting the slice to the "threshold value BASE.sub.b " and an inclination to "SLOPE.sub.b ". (Hereinafter, the function f(x) having the above characteristics is shown by "f.sub.b (x)".)
Therefore, when the above equation (1) is executed on the assumption that the original pixel value S.sub.org is set to the density correspondence amount, an effect for an image such that a density is reduced at a position where the average density of the image is high is derived.
By using the method by the "self compensation digital filter" as mentioned above to, for example, the mediastinum image where it is very difficult for the x-ray to transmit, a density of the mediastinum region of the x-ray chest image increases in accordance with the characteristics shown in FIG. 1A, so that the x-ray chest image by which both of the lung sac and the mediastinum can be observed can be obtained.
Besides the method by the foregoing self compensation digital filter, there is also a method of compressing a dynamic range due to a difference of characteristic amounts of anatomical regions by using a result of an anatomical segmentation.
That is, according to the above method (hereinafter, referred to as a method by the dynamic range compression), as disclosed in detail in ("Anatomic Region Based Dynamic Range Compression for Chest Radiographs Using Warping Transformation of Correlated Distribution", SPIE Medical Imaging 97), in an x-ray chest image, a mediastinum region is defined by executing a predetermined image process from a result obtained by classifying and extracting (hereinafter, referred to as "segmentation") the lung sac region, an affine transforming function for transforming a pixel value is automatically determined for the lung sac region and/or the mediastinum region, and distributions of the pixel values in the two image areas of the lung sac region and the mediastinum region and average values of their peripheral pixels are analyzed.
Even when the method by the dynamic range compression as mentioned above is used for, for example, the image of the mediastinum where it is very difficult for the x-ray to transmit, the density of the mediastinum region of the x-ray chest image increases. The x-ray chest image by which both of the lung sac and the mediastinum can be observed can be obtained.
However, in the case where the method by the "self compensation digital filter" or the method by the dynamic range compression as mentioned above is used for, for example, the image of the mediastinum where it is very difficult for the x-ray to transmit, although the density of the mediastinum region increases in the x-ray chest image and the anatomical line can be easily seen, there is a problem such that if the density of the mediastinum region is excessively raised, the contrast of the whole image including the lung sac region deteriorates.
When such an x-ray chest image is observed, an impression as if the images were overlapped is given and it is very hard for the doctor or the like to see and diagnose.
When the dynamic range compression is executed, further, since a step of deciding parameters which are necessary for such compression is not clear, a stable x-ray chest image cannot be derived.